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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

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based on the difference in the melting points of (i) the solid glycerides
recrystallised from ether and (2) the fatty acids obtained from them.

(a)  Preparation of the glycerides.    In a beaker of about 150 c.c. capacity,
50 grams of the fused and filtered fat are dissolved in 50 c.c. of ether, the
beaker being covered with a clock-glass, cooled to 15 and, with frequent
shaking, allowed to crystallise.    After an hour the mass is filtered through
a funnel containing a perforated disc covered with a layer of filter-paper
pulp, the liquid being pumped off and the crystalline mass left in the funnel
then pressed with a watch-glass to free it from mother-liquor.    The mass is
then dissolved again in 50 c.c. of ether and after an hour filtered off as btfore.

The melting point of the glycerides thus obtained is usually 63-64
for pure lard, but lower if tallow is present. If the melting point is below
61, the glycerides must be recrystallised from ether in the manner described
until a portion melting at least as high as 61 is obtained. To judge with
certainty, it is necessary that the glycerides melt between 61 and 65.

To obtain a good crystallisation of the solid glycerides in the case of
soft fats rich in liquid glycerides, the ethereal solution should be cooled
to 5-10, or use made either of a mixture of 3-4 parts of ether with I part
of alcohol, or of anhydrous acetone.

(b)  Preparation of the fatty acids.    From c-i to 0-2 gram of the glycerides,
m.p. 61-65, is finely subdivided and placed in a beaker with 10 c.c. of about
seminormal colourless  alcoholic  solution  of potassium  hydroxide.    The
liquid is boiled carefully for 5-10 minutes to bring about saponification,
the soap being dissolved in 100 c.c. of water and the solution transferred
to a separating funnel, decomposed with 2-3 c.c. of 25% hydrochloric
acid and extracted with 25 c.c. of ether.   The filtered ethereal solution is
evaporated and the residue dried at about 100 for 30-60 minutes and, when
cold, finely powdered.

(c)  Determination of the melting points.    The  melting points  of the
glycerides and of the fatty acids prepared according to (a) and (b) are now
determined under identical conditions.    For this purpose, two very thin,
perfectly similar U-tubes are used.   With the help of a platinum wire, the
finely powdered substance is introduced into one of the limbs of the U-tube
so as to form a layer 2-3 mm. deep.   The two tubes are then attached to
a thermometer so that the branches containing the substances adhere to
the bulb and the whole heated in a water-bath;   when a temperature of
50 is reached, the heating is adjusted so that the rise in temperature is
only 1-5-2 per minute.   The temperature at which the layer becomes
liquid, clear and transparent is taken as the melting p.oint.    The deter-
mination should be repeated with fresh substance and the mean of two
concordant results taken.

With pure hog's fat the difference (d) between the melting points of
the glycerides (M.G.) and of the fatty acids (M.A.), for values of M.G.
lying between 61 and 65, is never less than the values in the following
table:e saccharose, but,